Pandemic 2009 H1N1 influenza is caused by a new strain of H1N1 influenza A virus, which genetically is a combination of swine, avian, and human influenza viruses.

Most often, pigs have been infected by strains of influenza that are slightly different from those that infect people. These strains very rarely spread to people, and when they do, they very rarely then spread from person to person. The H1N1 swine flu virus is a combination of swine, avian, and human influenza viruses that spreads easily from person to person. The infection is not acquired through ingestion of pork and is acquired very rarely by contact with infected pigs.

In June 2009, the World Health Organization declared H1N1 swine flu a pandemic; it spread to > 70 countries and to all 50 US states. The majority of the deaths initially occurred in Mexico. The attack rate and mortality for H1N1 swine flu are higher in young and middle-aged adults and lower in the elderly than they are for seasonal flu. The pandemic entered the post-pandemic period in August 2010. Subsequently, the virus name was standardized to influenza A(H1N1)pdm09 to denote the pandemic and distinguish the virus from seasonal H1N1 strains and the 1918 pandemic H1N1 strain.

Human cases of H3N2 virus infection have occurred sporadically in several US states where children and adults have had contact with apparently healthy domestic pigs at agricultural fairs There have also been cases of possible human-to-human transmission. The H3N2 virus has genes from avian, swine, and human viruses and the matrix (M) gene from the A(H1N1)pdm09 virus.

Symptoms and Signs

Symptoms, signs, and complications resemble those of ordinary influenza (see Influenza : Symptoms and Signs), although nausea, vomiting, and diarrhea may be more common. Symptoms are usually mild, but they can become severe, leading to pneumonia or respiratory failure. Currently circulating isolates appear to have lost some of their initial virulence.

Diagnosis

Sometimes PCR testing of respiratory samples

Because A(H1N1)pdm09 swine flu is the predominant strain of influenza currently circulating worldwide, this diagnosis should be considered in any patient with influenza-like symptoms.

A PCR test can detect the A(H1N1)pdm09 virus in respiratory tract samples (eg, nasopharyngeal swabs, nasal washings, tracheal aspirates). Mildly ill patients do not require testing other than for epidemiologic or surveillance purposes; however, local hospital and public health requirements may vary. Rapid antigen detection tests have decreased sensitivity and generally are clinically useful in diagnosis only if results are positive.

Treatment

Sometimes a neuraminidase inhibitor

Treatment focuses mainly on symptom relief (eg, acetaminophen or ibuprofen for fever and aches). Antiviral drugs may be used, particularly for high-risk patients (see Influenza : At-risk groups) and those who are seriously ill. Oseltamivir and zanamivir appear to be effective; they are most effective when started within 48 h after symptom onset. In the US, the FDA has issued Emergency Use Authorizations for the use of oseltamivir in patients < 1 yr old and for the emergency use of peramivir, an IV neuraminidase inhibitor, in severely ill hospitalized patients.

Most patients recover fully without taking these drugs.

Prevention

The current seasonal influenza vaccines are effective against the A(H1N1)pdm09 virus.

Commonsense steps (eg, staying home if influenza-like symptoms develop; thorough, frequent handwashing with soap and water or an alcohol-based hand sanitizer) are recommended to reduce the spread of infection.

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